One type of balancing chamber used in dialysate supply machines has rigid outer walls and a diaphragm that divides the chamber into two regions so that as one region is being filled with fresh dialysate the other is discharged of an equal amount of spent dialysate as the diaphragm moves toward one of the rigid walls. When all of the spent dialysate has been discharged from the spent dialysate region, the valves at inlets and outlets to the regions are switched, and the spent dialysate side is filled, as the fresh dialysate side discharges, and the diaphragm moves toward the other wall, until all fresh dialysate has been discharged, and so on.
It is desirable to accurately sense when the diaphragm approaches a wall so that the valves to the chamber switch from one mode to the other at the proper time. Schal U.S. Pat. No. 4,530,759, which is hereby incorporated by reference, discloses sensing when a diaphragm has reached a wall by sensing when a pump supplying dialysate to a region is drawing a sharply increasing electrical current. Flowrate sensors, pressure sensors, and contact switches have been suggested as other mechanisms to sense the end of a stroke in similar applications in Papanek et al. U.S. Pat. No. 4,366,061 (col. 9, lines 40-47); Pinkerton U.S. Pat. No. 4,178,240 (col. 3, lines 18-24) and Schal U.S. Pat. No. 4,267,040 (col. 6, lines 29-47).